Phase transitions in LaFeAsO: structural, magnetic, elastic, and transport properties, heat capacity and Mossbauer spectra

TL;DR

This paper provides a comprehensive experimental study of LaFeAsO, revealing its structural, magnetic, elastic, and transport phase transitions around 160-145 K, and explores their effects on various physical properties.

Contribution

It offers detailed insights into the phase transitions and their impact on LaFeAsO's properties, including structural, magnetic, and transport phenomena, using multiple experimental techniques.

Findings

Structural transition occurs at ~160 K, with magnetic ordering near 145 K.

Magnetic moment of ~0.35 Bohr magnetons per iron in low temperature phase.

Structural distortions influence carrier localization and magnetic ordering.

Abstract

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling this material undergoes a tetragonal-orthorhombic crystallographic phase transition at ~160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder X-ray and neutron diffraction measurements is presented. A magnetic moment of ~0.35 Bohr magnetons per iron is derived from Mossbauer spectra in the low temperature phase. Evidence of the structural transition is observed at temperatures well above the structural transition (up to near 200 K) in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties (resistivity, thermal conductivity, Seebeck coefficient, Hall coefficient), heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.